It is common prior art practice to seal a shaft with a packing arrangement formed by a plurality of stacked V-shaped rings. The V-shaped rings are generally disposed in the packing chamber of a structural body in a circumferentially sealing relationship tot he shaft. Follower rings (also known as adaptor assemblies) are typically employed with the V-shaped rings and are positioned on opposite axial sides of the V-shaped rings. The externally positioned follower ring is urged into axially compressive engagement with the stacked V-shaped rings by a packing gland, with the internally positioned follower rings restrained from axial movement by the bottom of the packing chamber. Axial compression of the V-shaped ring stack tends to radially expand the rings and to assist in maintaining a sealing relationship between the V-shaped rings and the shaft.
Such V-shaped rings have been formed from a wide variety of materials. For example, elastomeric V-shaped rings formed from homogeneous rubbers have been used in many applications where relatively low pressures are encountered and where the fluid media involved does not damage the rubber. Fabric V-shaped rings coated with elastomers are often used on heavy duty equipment or when higher pressures are encountered. When corrosive media is handled, however, the corrosive media will often attack both the rubber and the fabric rings, making them unsuitable for commercial use. Under such conditions, prior art workers have used V-shaped rings of a molded fluoronated hydrocarbon polymer, such as polytetrafluoroethylene. Many fluoronated hydrocarbon polymers, such as polytetrafluoroethylene, are inert to virtually all chemical media and are suitable for use with a wide variety of corrosive fluids. In addition, fluoronated hydrocarbon polymeric V-shaped rings operate successfully through a wide range of temperatures, from approximately -120.degree. F. to about 350.degree. F. Many of the fluoronated hydrocarbon polymers are characterized by extremely low coefficience of friction.
A problem arises form the fact that fluoronated hydrocarbon polymers have a high coefficient of expansion relative to most metals. In addition, when cooled after exposure to elevated temperatures, fluoronated hydrocarbon polymers may shrink to a size which is smaller than their original size. As a result, even when prior art polytetrafluoroethylene packing rings are initially compressed tightly in sealing relationship about a metal shaft in a metal structural body, the sealing relationship may be lost if the system is thermally cycled.
Prior art polytetrafluoroethylene seals have been formed to the desired V-shaped configuration by compressive molding techniques. Such molding techniques, however, require the rings to have a minimum thickness of about 1/8 inch. This thickness requirement, when the rings are stacked in a packing arrangement, limits the minimum stack height requirement and thus limits the number of independent sealing surfaces acting on the shaft and packing chamber sidewall. Furthermore, compressively molded rings are permanently shaped and normally are compressed to form sealing surfaces with the shaft and packing chamber sidewalls.
U.S. Pat. No. 4,512,586 addressed these problems by forming a plurality of packing rings from memory retainable polytetrafluoroethylene. The rings are initially flat and are given a pre-formed V-shaped cross-sectional configuration in a forming dye. The pre-formed V-shaped rings are disposed in aligned relationship in the packing chamber between the cylindrical sidewall thereof and the shaft with the packing ring sidewalls compressed toward each other beyond the pre-formed V-shaped configuration. The memory retainable material is operative to urge the rings toward their original horizontal configuration whereby the sealing relationship between the cylindrical sidewall of the packing chamber and the shaft is enhanced, and is maintained despite thermocycling.
While such dye formed rings, taking advantage of the memory retainable material from which they are made, represent an improvement in the art, it has been found that whenever leakage did occur, it most often occurred between the rings and the cylindrical sidewall of the packing chamber. It has further been found that the memory of most fluoronated hydrocarbon polymers is very good in tension, but not as good in compression.
U.S. Pat. No. 4,333,632 teaches a sealing assembly comprising a plastic diaphragm, a delta ring, a metal diaphragm, and a floating thrust collar. The plastic diaphragm is made of a fluorinated hydrocarbon polymer and has a preformed tapered sealing lip which is deformed into a substantially cylindrical shape by the shaft and delta ring with which it makes a seal. While the seal disclosed in this patent has some tension sealing, the seal area is relatively wide and the seal is basically a compressive seal.
The present invention teaches sealing means made of memory retainable fluoronated hydrocarbon polymer and configured to prevent leakage between the sealing means and the surrounding body by means of a gasket seal. The sealing means of the present invention also provides at least one surface which is distorted and stretched into a tapered configuration and into a tension-related sealing association with the shaft. This tension-related narrow sealing surface is enhanced by the memory of the material of which the sealing means is made and is further enhanced by the pressure of the media. A sealing relationship with the shaft is maintained even in a system subject to thermocycling. It has also been found that when the shaft shifts with respect to the surrounding body due to the media pressure, resulting in misalignment, a tension seal flexes and maintains its seal, while a compression seal would not.